Measure the roof’s horizontal footprint, including the overhangs you need to cover, then choose the pitch factor and your waste allowance. We’ll estimate the roof area and material bundles.
Valleys, hips, dormers, starter strips and ridge pieces can change the order. Confirm the final layout and product coverage with your roofer; there is no need to climb onto the roof to use this tool.
How to use this calculator
- Measure the building footprint including the eaves overhang — shingles cover the overhang too.
- Find the pitch without climbing on the roof: from a rafter in the loft, or a pitch-finder app against the gable end.
- Select the matching multiplier and your shingle's bundles-per-square figure from its specification sheet.
- Add waste for your roof shape and round up to whole bundles; ridge cap shingles are a separate line item.
How shingle quantities are calculated
True roof area = footprint area x pitch multiplier, where the multiplier is the square root of (1 + (rise/12) squared). Squares = area including waste / 100. Bundles = squares x bundles per square — three for most standard architectural shingles. The multiplier is why a 12/12 (45 degree) roof has about 41 percent more surface than its footprint.
A 30 ft x 20 ft house footprint with a 6/12 pitch has 600 x 1.118 = 671 sq ft of roof. With 10 percent waste that is 738 sq ft, or 7.38 squares — 23 bundles of standard shingles, plus ridge caps.
A few things worth checking
- Ordering from the footprint area alone and under-buying by the entire pitch factor — 12 percent on a 6/12 roof, 41 percent on a 12/12.
- Forgetting ridge and hip cap shingles, starter strips and underlayment, which are separate products with separate counts.
- Underestimating waste on roofs with valleys and dormers; every valley is a line of cut shingles on both sides.
Your questions, answered
How many bundles of shingles are in a square?
A roofing square is 100 sq ft. This tool lets you select 3 or 4 bundles per square; it multiplies the area including waste by that choice and rounds up. Use the coverage specified for your exact shingles.
How is roof pitch used in this estimate?
Pitch is rise over horizontal run. The tool applies the multiplier beside your selected pitch to the entered footprint area. For example, the 6/12 option uses 1.118. It does not measure the roof or model several different pitches at once.
Does this calculate the full roofing material order?
No. It estimates the main shingle quantity from footprint, pitch and waste. Ridge or hip caps, starter strips, flashing and other details need separate counts. It does not decide whether an existing roof can be overlaid.
How is underlayment estimated?
The tool divides the roof area including waste by an assumed 400 sq ft per roll and rounds up. It does not lay out overlaps or special membranes. Check the actual roll coverage and installation details for your roofing system.
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